Autoimmune Addison’s disease (AAD) is a rare and highly heritable endocrinopathy. It is a complex genetic disease, meaning that it is due to a combination of interacting environmental and genetic factors. To date, the majority of the substantial genetic component to AAD aetiology remains undefined. In this study, a combination of hypothesis-driven (candidate gene) and discovery-driven (genome-wide) approaches have been used to search for novel genetic determinants of AAD. PCR-based approaches were undertaken to study the potential role of the CYP21A1P pseudogene in AAD. CYP21A1P is highly homologous to the CYP21A2 gene which encodes 21-hydroxylase, the primary autoantigen in AAD. In individuals with AAD, CYP21A1P is more likely to be absent from the genomic DNA sequence than in controls. qPCR and in situ hybridisation have been successfully combined to identify CYP21A1P transcripts in thymic, and fetal adrenal, tissue. These data perhaps indicate a role for CYP21A1P in induction of immune tolerance, with its loss being associated with autoimmunity against the steroidogenic apparatus. Taking a broader candidate gene approach, the largest association analysis in AAD to date, of twenty candidate genes in six European AAD cohorts, suggests a role for NF-κB1, IL23A and GATA3 variants in susceptibility to AAD in individual European cohorts, and a role for STAT4 more universally in AAD. SNP array technology has been used to conduct the first genome-wide linkage and association analysis in AAD. The linkage study, including 23 families, has linked regions on chromosomes 6, 7, 9 and 18 to disease. A genome-wide association analysis, comparing the 50 familial AAD cases to the Wellcome Trust 1958 UK Birth Cohort control group, revealed clusters of associated SNPs on chromosomes 2 and 6. iii This body of work has illustrated some of the challenges in investigating a rare, complex genetic disorder, and how international collaboration can help to resolve some of these issues. In the course of this work, in addition to identifying a number of novel genetic determinants to AAD, exciting preliminary results have been generated which will need to be followed up. It is hoped that once these preliminary findings are replicated and further investigated, they will contribute significantly to an increase in our understanding of the pathogenesis of AAD, with the long-term aim of identifying novel means of treating the disease, altering its natural history or even preventing it.